1. Field of the Invention
The present invention relates generally to micro-fluid ejection, devices and, more particularly, to a planar heater stack and a method for making the planar heater stack with a cavity within the planar heater substrata above the substrate thereof.
2. Description of the Related Art
Micro-fluid ejection devices have had many uses for a number of years. A common use is in a thermal inkjet printhead in the form of a heater chip. In addition to the heater chip, the inkjet printhead basically includes a source of supply of ink, a nozzle plate attached to or integrated with the heater chip, and an input/output connector, such as a tape automated bond (TAB) circuit, for electrically connecting the heater chip to a printer during use. The heater chip is made up of a plurality of resistive heater elements, each being part of a heater stack. The term “heater stack” generally refers to the structure associated with the thickness of the heater chip that includes first, or heater forming, strata made up of resistive and conductive materials in the form of layers or films on a substrate of silicon or the like and second, or protective, strata made up of passivation and cavitation materials in the form of layers or films on the first strata, all fabricated by well-known processes of deposition, patterning and etching upon the substrate of silicon. The heater stack also has one or more fluid vias or slots that are cut or etched through the thickness of the silicon substrate and the first and second strata, using these well-known, processes, and serve to fluidly connect the supply of ink to the heater stacks. A heater stack having this general construction is disclosed as prior art in U.S. Pat. No. 7,195,343, which patent is assigned to the same assignee as the present invention. The disclosure of this patent is hereby incorporated by reference herein.
Despite their seeming simplicity, construction of heater stacks requires consideration of many interrelated factors for proper functioning. The current trend for inkjet printing technology (and micro-fluid ejection devices generally) is toward lower jetting energy, greater ejection frequency, and in the case of printing, higher print speeds. A minimum quantity of thermal energy must be present on an external surface of the heater stack, above a resistive heater element therein, in order to vaporize the ink inside an ink chamber between the heater stack external surface and a nozzle in the nozzle plate so that the ink will vaporize and escape or jet through the nozzle in a well-known manner. The overall heating energy or “jetting energy” produced by the heater stack must pass through the plurality of layers of the first and second strata that form the heater stack before the requisite energy for fluid ejection reaches the external surface of the heater stack. The greater the thickness of the layers of the first strata of the heater stack, the more jetting energy that will be required before the requisite energy for ink drop formation and ejection can be reached on the heater stack external surface. However, a minimum presence of protective layers of the second strata of the heater stack is necessary to protect the resistive heater element from chemical corrosion, and from mechanical stress from the effects of cavitation.
During inkjet heater chip operation, some of the heating energy is wasted due to heating up the “heater overcoat”, or the second strata, and also heating up the substrate. Since heating or jetting energy required is proportional to the volume of material of the heater stack that is heated during an ejection sequence, reducing the heater overcoat thickness, as proposed in U.S. Pat. No. 7,195,343 is one approach to reducing the jetting energy required. However, as the overcoat thickness is reduced, corrosion of the ejectors or heater elements becomes more of a factor with regard to ejection performance and quality. Another issue associated with a decrease in overcoat thickness is that heat loss to the substrate becomes a larger factor.
Thus, there is a need for an innovation that will reduce the heat loss to the substrate.